Phenoxyalkanoic acids and their salts and esters are well-known herbicides, especially those wherein the phenyl radical bears halogen substituents. The 2,4-dichloro- and 2,4,5-trichlorophenoxyacetic and -propionic acids and their derivatives are typical examples.
The phenoxyalkanoic acids are usually made by the reaction of an alkali metal salt of the corresponding phenol with an alkali metal salt of the appropriate haloalkanoic acid. These salts are usually made by reaction of the phenol and the acid with alkali metal hydroxide, thus producing water as a by-product. Additionally, water is often used as a solvent for the reaction.
An unavoidable by-product of the above process is hydroxyalkanoic acid. It is formed by the hydrolysis of the haloalkanoic acid and is highly objectionable, both because of the loss of haloalkanoic acid and because of the effort and expense entailed in separating it from the product and from the waste stream of the process. The latter is essentially an aqueous brine and presents a disposal problem. If sufficiently pure, however, it can be electrolyzed to produce halogen and alkali metal hydroxide, both of which can be recycled to the process.
Prior processes have usually attempted to minimize the production of hydroxy acid by minimizing the amount of water present. This has been done by use of a two-phase water-organic reaction medium. U.S. Pat. No. 2,480,817 is typical of this approach.
Another technique involves the use of excess salt of the phenol. To this is added the haloalkanoic acid per se rather than its salt (U.S. Pat. No. 2,651,659).
In still another modification of the above process, excess phenol is used as a liquefying medium in combination with another organic solvent (U.S. Pat. No. 2,656,382). In this process, the amount of water present in the reaction mixture was minimized by using substantially anhydrous reactants. While this process apparently produces high yields, it is tedious and expensive because
1. it requires large volumes of organic solvent, PA1 2. it requires the preparation of anhydrous alkali metal salt of the haloalkanoic acid, and PA1 3. it requires the metering into the reactor of the solid salt or of a slurry thereof in an organic liquid, which liquid must later be separated and recycled. PA1 a. a solution or dispersion of alkali metal phenate in the corresponding phenol, PA1 b. alkali metal hydroxide and PA1 c. haloalkanoic acid,
The preparation of the anhydrous salt of the haloalkanoic acid is an especially troublesome problem because of the ease with which the halogen is removed by hydrolysis, thus producing the undesirable hydroxyalkanoic acid salt. Thus, the neutralization of the acid with caustic must be conducted very carefully at subnormal temperatures and the evaporation of the water therefrom must be done under vacuum at low temperatures to avoid hydrolysis.
In view of the above problems, it is an object of this invention to provide a process for making phenoxyalkanoic acids that does not require the use of an extraneous organic solvent as the reaction medium and that does not require the prior preparation of the alkali metal haloalkanoate. Other objects will appear hereinafter.